x86/pvops: add a paravirt_ident functions to allow special patching

Impact: Optimization

Several paravirt ops implementations simply return their arguments,
the most obvious being the make_pte/pte_val class of operations on
native.

On 32-bit, the identity function is literally a no-op, as the calling
convention uses the same registers for the first argument and return.
On 64-bit, it can be implemented with a single "mov".

This patch adds special identity functions for 32 and 64 bit argument,
and machinery to recognize them and replace them with either nops or a
mov as appropriate.

At the moment, the only users for the identity functions are the
pagetable entry conversion functions.

The result is a measureable improvement on pagetable-heavy benchmarks
(2-3%, reducing the pvops overhead from 5 to 2%).

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>

1 files changed, 66 insertions, 9 deletions

diff --git a/arch/x86/kernel/paravirt.c b/arch/x86/kernel/paravirt.c
index 202514be5923..dd25e2b1593b 100644
--- a/arch/x86/kernel/paravirt.c
+++ b/arch/x86/kernel/paravirt.c
@@ -44,6 +44,17 @@ void _paravirt_nop(void)
 {
 }
 
+/* identity function, which can be inlined */
+u32 _paravirt_ident_32(u32 x)
+{
+        return x;
+}
+
+u64 _paravirt_ident_64(u64 x)
+{
+        return x;
+}
+
 static void __init default_banner(void)
 {
         printk(KERN_INFO "Booting paravirtualized kernel on %s\n",
@@ -138,9 +149,16 @@ unsigned paravirt_patch_default(u8 type, u16 clobbers, void *insnbuf,
         if (opfunc == NULL)
                 /* If there's no function, patch it with a ud2a (BUG) */
                 ret = paravirt_patch_insns(insnbuf, len, ud2a, ud2a+sizeof(ud2a));
-        else if (opfunc == paravirt_nop)
+        else if (opfunc == _paravirt_nop)
                 /* If the operation is a nop, then nop the callsite */
                 ret = paravirt_patch_nop();
+
+        /* identity functions just return their single argument */
+        else if (opfunc == _paravirt_ident_32)
+                ret = paravirt_patch_ident_32(insnbuf, len);
+        else if (opfunc == _paravirt_ident_64)
+                ret = paravirt_patch_ident_64(insnbuf, len);
+
         else if (type == PARAVIRT_PATCH(pv_cpu_ops.iret) ||
                  type == PARAVIRT_PATCH(pv_cpu_ops.irq_enable_sysexit) ||
                  type == PARAVIRT_PATCH(pv_cpu_ops.usergs_sysret32) ||
@@ -373,6 +391,45 @@ struct pv_apic_ops pv_apic_ops = {
 #endif
 };
 
+typedef pte_t make_pte_t(pteval_t);
+typedef pmd_t make_pmd_t(pmdval_t);
+typedef pud_t make_pud_t(pudval_t);
+typedef pgd_t make_pgd_t(pgdval_t);
+
+typedef pteval_t pte_val_t(pte_t);
+typedef pmdval_t pmd_val_t(pmd_t);
+typedef pudval_t pud_val_t(pud_t);
+typedef pgdval_t pgd_val_t(pgd_t);
+
+
+#if defined(CONFIG_X86_32) && !defined(CONFIG_X86_PAE)
+/* 32-bit pagetable entries */
+#define paravirt_native_make_pte        (make_pte_t *)_paravirt_ident_32
+#define paravirt_native_pte_val         (pte_val_t *)_paravirt_ident_32
+
+#define paravirt_native_make_pmd        (make_pmd_t *)_paravirt_ident_32
+#define paravirt_native_pmd_val         (pmd_val_t *)_paravirt_ident_32
+
+#define paravirt_native_make_pud        (make_pud_t *)_paravirt_ident_32
+#define paravirt_native_pud_val         (pud_val_t *)_paravirt_ident_32
+
+#define paravirt_native_make_pgd        (make_pgd_t *)_paravirt_ident_32
+#define paravirt_native_pgd_val         (pgd_val_t *)_paravirt_ident_32
+#else
+/* 64-bit pagetable entries */
+#define paravirt_native_make_pte        (make_pte_t *)_paravirt_ident_64
+#define paravirt_native_pte_val         (pte_val_t *)_paravirt_ident_64
+
+#define paravirt_native_make_pmd        (make_pmd_t *)_paravirt_ident_64
+#define paravirt_native_pmd_val         (pmd_val_t *)_paravirt_ident_64
+
+#define paravirt_native_make_pud        (make_pud_t *)_paravirt_ident_64
+#define paravirt_native_pud_val         (pud_val_t *)_paravirt_ident_64
+
+#define paravirt_native_make_pgd        (make_pgd_t *)_paravirt_ident_64
+#define paravirt_native_pgd_val         (pgd_val_t *)_paravirt_ident_64
+#endif
+
 struct pv_mmu_ops pv_mmu_ops = {
 #ifndef CONFIG_X86_64
         .pagetable_setup_start = native_pagetable_setup_start,
@@ -424,21 +481,21 @@ struct pv_mmu_ops pv_mmu_ops = {
         .pmd_clear = native_pmd_clear,
 #endif
         .set_pud = native_set_pud,
-        .pmd_val = native_pmd_val,
-        .make_pmd = native_make_pmd,
+        .pmd_val = paravirt_native_pmd_val,
+        .make_pmd = paravirt_native_make_pmd,
 
 #if PAGETABLE_LEVELS == 4
-        .pud_val = native_pud_val,
-        .make_pud = native_make_pud,
+        .pud_val = paravirt_native_pud_val,
+        .make_pud = paravirt_native_make_pud,
         .set_pgd = native_set_pgd,
 #endif
 #endif /* PAGETABLE_LEVELS >= 3 */
 
-        .pte_val = native_pte_val,
-        .pgd_val = native_pgd_val,
+        .pte_val = paravirt_native_pte_val,
+        .pgd_val = paravirt_native_pgd_val,
 
-        .make_pte = native_make_pte,
-        .make_pgd = native_make_pgd,
+        .make_pte = paravirt_native_make_pte,
+        .make_pgd = paravirt_native_make_pgd,
 
         .dup_mmap = paravirt_nop,
         .exit_mmap = paravirt_nop,